This invention relates to dual beam laser devices, and, in particular relates to an apparatus used in the optical train thereof for controlling the angular separation of a pair of crossing laser beams.
One type of apparatus for causing parallel beams to cross is a conventional lens. One beam is input along the optical axis and crosses the focal point on the output side of the lens thereby. The other beam is input parallel to the optical axis but offset therefrom and as a result of the lens crosses the focal point also. The angular separation of the beams is determined by the perpendicular distance separating the input parallel beams and the focal length of the lens. To minimize the angular separation requires a very long focal length lens.
There are several disadvantages of a simple lens. The first disadvantage is that the lens has a fixed focal length and thus the crossing point cannot be varied irrespective of other factors. Additional lenses could be added but these would increase the optical path length, distortion, and loss of intensity. Another disadvantage is the lens has a thickness which inherently produces optical distortion on the output side. As related to the first disadvantage, one typically desires to minimize the length of any optical train because of distortion and vibrations in the components. Thus to reduce length, one would have to use short focal length lenses but when this is done the distortion is greatly increased because of the lens thickness and angular separation of the output beams at the focal point is increased. This increases the diameter of the aperture at the crossing point. Thus the desire to minimize angular separation and to minimize optical train length are clearly not realizable with the above apparatus.
These drawbacks have motivated a search for an apparatus that is able to minimize angular separation of the output beams and minimize the length of the optical train required to cross two parallel beams with minimum separation but not being coincident.